Radar altimeter

ABSTRACT

A radar system operates at a carrier frequency near the oxygen absorption line. A selected range bin is monitored to measure the signal-to-noise ratio. The signal-to-noise ratio value is used to adjust the carrier frequency so as to maintain a preselected signal-to-noise ratio.

CROSS-REFERENCE TO RELATED APPLICATION

This Reissue application is a reissue of application Ser. No.06/572,286, filed Jan. 20, 1984, which is incorporated by referenceherein.

This invention is concerned with radar type altimeters. Specifically,the radar altimeter of the present invention is one which maintains asignal to noise level below a value which affords detection by anintercept receiver.

Radar altimeters are well known. In order to provide covert operation,i.e. not detectable by an intercept receiver, the radar signal should bea minimum to reduce the chances of radar detection by the interceptreceiver.

The novel radar system of the present invention provides a radaraltimeter which operates near the oxygen absorption line. The frequencyof the transmitter is adjusted in accordance with a measurement of thesignal to noise ratio of a preselected range bin (return signal) therebyminimizing detection by an intercept receiver.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graphical representation of signal absorption versusfrequency.

FIG. 2 is a diagrammatic representation of intercept receiver signal tonoise ratio versus range measured from the transmitter.

FIG. 3 is a block diagram of one embodiment of the invention of thepresent application.

As shown in FIG. 1, atmospheric attenuation of a radar signal is near amaximum when the transmitter frequency is at 60 GHz. It should be notedthat FIG. 1 is presented as a representation and is not intended to be adetailed and exact graph of frequency versus absorption. Nevertheless,it can be seen that a change of 8 DB/KM (one way) attenuation occurs bychanging the radar frequency from 53 GHz to 57 GHz. Thus, large changesin attenuation are obtainable from small variations in the transmitterfrequency, i.e. 4 GHz.

FIG. 2 shows a diagrammatic representation of intercept receiver signalnoise ratio versus range from the transmitter. FIG. 2 illustrates thedifferences between signal noise ratio for microwave signals 20 and GHzsignals 25. A constant signal to noise ratio of 15 DB line is shown onthe graph. A microwave transmitter can be detected at 15 dB signal tonoise ratio at 800 K feet. However, at a frequency in the order of 55GHz, this detection range is reduced to about 60 K feet (10 nauticalmiles). As noted in FIG. 2, by automatically shifting the transmitterfrequency by 1 GHz, the intercept receiver range reduces to less than 20K feet. In other words, the intercept detection range is reduced by over40 times as compared with a microwave system.

Shown in FIG. 3 is a schematic block diagram which illustrates theprinciples of the invention of the present application. Oscillator 300provides a carrier signal to be transmitted by transmitter 305 forsubsequent transmission through antenna 310 via duplexer 315. Oscillator300 includes a control input 301 to vary the carrier signal frequency tosome extent.

By way of example, the radar system shown in FIG. 3 is an altimeter.Altimeter return signals are received by antenna 310 and processedthrough duplexer 315 which feeds receiver 320. The output from receiver320 is fed into signal-to-noise detector 340.

The output of detector 340 is presented to a hold circuit 342 forholding the value of the signal-to-noise ratio upon a predetermined binselection provided by the output of bin selector 345. The output of holdcircuit 342 is then presented as one input to comparator 350. Comparator350 compares the output of hold circuit 342 to a preselected referencelevel input. The comparator provides an output signal to integrator 360which integrates the output signal of comparator 350. The output ofintegrator 360 is then presented to the control input 301 of oscillator300 for adjusting the oscillation frequency, i.e. the carrier frequencyin proportion to the output of integrator 360.

The operation of the radar system of FIG. 3 will now be described. Thereference signal is a signal representative of a predeterminedacceptable signal-to-noise ratio which affords only limited detection byan intercept receiver at a predetermined range. Oscillator 300 providesa signal in the order of, for example, 55 GHz which is subsequentlytransmitted over antenna 310 through transmitter 305 and duplexer 315.Return signals, in time, are operated on by receiver 320 for usualaltimeter range determination. Receiver 320 provides the usualamplification and detection circuits necessary for standard altimeters.An output of receiver 320 is provided to detector 340 which providessignal-to-noise detection in the usual manner well known in the art ofradio signals. The output of detector 340, a signal indicative of thesignal-to-noise ratio, is presented to hold circuit 342 which will holdthe value of the signal-to-noise detector 340 when the holding circuit342 is gated by bin selector 345. Gating occurs upon a predeterminedrange which usually would be the maximum range desired in the operatingsystem.

The output of the holding circuit 342 is then presented to comparator350 which is compared with the reference signal indicative of thepreselected acceptable signal to noise ratio. The output of thecomparator is then integrated via integrator 360 and presented to theadjustable oscillator 300 for adjusting the oscillation so as toincrease or decrease as the case may be to maintain the preselectedsignal to noise ratio. The combination of comparator 350 and integrator360 provides a negative feedback control means so as to maintain thesignal-to-noise ratio as aforesaid.

It will be appreciated by those skilled in the art that the embodimentshown in FIG. 2 is applicable to a wide range of radar systems includingFM/CW, pulsed radar systems, and the like. Such systems are envisionedto be within the scope of the present application and arediagrammatically represented by a transmitter block 305. Similarly,dependent upon the system employed, receiver 320 is constructed in themanner to be compatible with the method of transmission by transmitter305.

The embodiments of the invention in which an exclusive property or rightis claimed are defined as follows:
 1. A radar system comprising:oscillator means for providing a high frequency signal having afrequency near the oxygen absorption frequency, said oscillator meanshaving means for adjusting said high frequency signal in response to acontrol signal; means for transmitting said high frequency signalthrough the atmosphere; means for receiving a reflected signal of saidtransmitted high frequency signal; means for detecting the asignal-to-noise ratio of said reflected high frequency signal at apreselected time after said transmission of said high frequency signal;means for comparing said signal-to-noise ratio with a preselected signalto noise ratio value and providing an output signal indicative of thedifference thereof; feedback control means for providing said controlsignal as a function of said comparator output signal.
 2. The radarsystem of claim 1 wherein said feedback control means includes meansresponsive to said comparator output signal for maintaining saidsignal-to-noise ratio substantially equal to said preselectedsignal-to-noise ratio.
 3. The radar system of claim 1, wherein the highfrequency signal is controlled using the control signal.
 4. The radarsystem of claim 1, wherein a nominal transmitted frequency of the highfrequency signal is from about 55 GHz to about 65 GHz.
 5. The radarsystem of claim 1, wherein the radar system is a radar altimeter.
 6. Aradar system comprising: an oscillator operative to generate a carriersignal, the oscillator including a control input; a transmitter inoperative communication with the oscillator, the transmitter configuredto transmit the carrier signal to atmosphere; a receiver operative toreceive a reflected return signal of the transmitted carrier signal; adetector in operative communication with the receiver, the detectorconfigured to determine a signal-to-noise ratio of the reflected returnsignal; and a comparator in operative communication with the detector,the comparator configured to compare the signal-to-noise ratio with apreselected signal-to-noise ratio value, and to provide an output signalindicative of the difference thereof; wherein a control signal, which isgenerated based on the output signal from the comparator, is sent to thecontrol input of the oscillator to adjust a frequency of the carriersignal.
 7. The radar system of claim 6, wherein the frequency of thecarrier signal is near an oxygen absorption frequency.
 8. The radarsystem of claim 6, further comprising: one or more antennas in operativecommunication with the transmitter and the receiver; a bin selector inoperative communication with the detector; a peak-hold detector inoperative communication with the bin selector, the peak-hold detectorconfigured to hold a value of the signal-to-noise ratio upon apredetermined bin selection provided by the bin selector; and anintegrator operative to receive the output signal from the comparator,the integrator configured to generate the control signal.
 9. The radarsystem of claim 8, wherein the one or more antennas are connected to thetransmitter and the receiver through a duplexer.
 10. The radar system ofclaim 8, wherein: the comparator is in operative communication with thepeak-hold detector and a preselected reference level input; and thereference level input comprises a signal representative of apredetermined acceptable signal-to-noise ratio which affords onlylimited detection by an intercept receiver at a predetermined range. 11.The radar system of claim 7, wherein the signal-to-noise ratio ismaintained below a value which affords detection by an interceptreceiver at a predetermined range.
 12. The radar system of claim 7,wherein the radar system comprises a radar altimeter.
 13. The radarsystem of claim 7, wherein the radar system comprises afrequency-modulated continuous-wave (FM/CW) radar system.
 14. The radarsystem of claim 7, wherein the radar system comprises a pulsed radarsystem.
 15. A method of operating a radar system, the method comprising:generating a carrier signal; transmitting the carrier signal through theatmosphere; receiving a reflected return signal of the transmittedcarrier signal; detecting a signal-to-noise ratio of the reflectedreturn signal at a preselected time after transmitting the carriersignal; comparing the signal-to-noise ratio of the reflected returnsignal with a preselected signal-to-noise ratio to generate an outputsignal indicative of the difference thereof; generating a control signalas a function of the output signal; and adjusting a frequency of thecarrier signal in response to the control signal using a feedbackcontrol loop.
 16. The method of claim 15, wherein the preselected timeafter transmitting the carrier signal is determined based on around-trip time delay associated with the reflected return signal. 17.The method of claim 15, wherein the preselected signal-to-noise ratio isdetermined based on an acceptably low probability of detection of thetransmitted carrier signal by an intercept receiver at a predeterminedrange.
 18. The method of claim 15, further comprising: monitoring aselected range bin to measure the signal-to-noise ratio of the reflectedsignal.
 19. The method of claim 15, further comprising controlling afrequency of the carrier signal such that the signal-to-noise ratio ofthe reflected return signal is substantially equal to the preselectedsignal-to-noise ratio.
 20. The method of claim 15, wherein the frequencyof the carrier signal is near an oxygen absorption frequency.